1. Field of the Invention
The present invention relates to a level shifter intervening between different voltage power-supply systems and to an analog-to-digital converter (referred to as A/D converter in this specification and claims) employing the level shifter.
2. Description of the Background Art
FIG. 9 is a block diagram showing the configuration of a conventional microcomputer 1. The microcomputer 1 comprises a CPU 2, an A/D converter 3, peripheral modules 7, 8, and an I/O interface 6. The A/D converter 3 comprises an analog section 4 and a digital section 5.
Power-supply VCC for digital circuit is applied to the CPU 2, the digital section 5 of the A/D converter 3, the I/O interface 6, and the peripheral modules 7, 8. Power-supply AVCC for analog circuit and a reference power-supply AVRFF provided in ladder resistors (not shown) that is normally equipped with the analog section 4, are applied to the analog section 4 of the A/D converter 3. A ground power-supply for digital circuit (not shown) is applied to the CPU 2, the digital section 5, and the peripheral modules 7, 8, respectively. A ground power-supply for analog circuit (not shown) is applied to the analog section 4.
There is, however, a first demand that the microcomputer 1 is protected from electromagnetic interference by preventing the switching noise of an output buffer (not shown) which is generally provided in the I/O interface 6 and the noise from the outside from being transferred to the inside. Therefore, the power-supply which is applied to the CPU 2, the digital section 5, and the peripheral modules 7, 8 is preferably provided separately from the power-supply applied to the I/O interface 6.
There is also a second demand that unnecessary radiation from the power dissipated by the CPU 2 and the peripheral modules 7, 8, and from an oscillation circuit (which is for example provided in the peripheral module 8 in order to produce a clock for controlling the operation of the microcomputer 1) is suppressed. It is therefore preferable that the voltage of power-supply applied to the CPU 2, the digital section 5, and the peripheral modules 7, 8, is lower than the voltage of power-supply applied to the I/O interface 6.
FIG. 10 is a block diagram showing a schematic configuration of a microcomputer 1 that has been improved based on the above stated thoughts. An internal logic circuit 14 indicates a collection of a CPU 2 and peripheral modules 7, 8. Power-supply VCCE for internal logic circuit is applied to the internal logic circuit 14. Power-supply VCCI for I/O interface is applied to an I/O interface 6. The power-supply VCCE for internal logic circuit and the power-supply VCCI for I/O interface apply a voltage VCCE and a voltage VCCI, respectively, which are different from each other (hereinafter the same symbol is used for a power-supply and the voltage applied by the power-supply). For instance, VCCI is set to 5 V, and VCCE is set to 3.3 V.
Since power-supply wires transmitting a different voltage are not connected to each other, the noise occurred in the I/O interface 6 or the noise from the exterior is not propagated to the internal logic circuit 14, avoiding the malfunction of the microcomputer 1. Thus, when systems are designed on user ends, it is unnecessary to take electromagnetic interference measures This satisfies the first demand. At the same time, it is possible to reduce power dissipation and to suppress the unnecessary radiation from the oscillation circuit provided in the peripheral module 8. This satisfied the second demand.
If the microcomputer 1 is configured by using a plurality of power-supplies differ in voltage system as discussed above, it is required to match the signal voltage between circuits operating in dissimilar power-supplies.
FIG. 11 shows a configuration which differs from the configuration of FIG. 10 in that a level shifter 11a for performing the level shift of a signal voltage traveling between an internal logic circuit 14 and an I/O interface 6 is added therebetween. Such techniques that the voltage of power-supply for an I/O circuit is different from that for a logic circuit, and the levels of signal voltages traveling between the two are matched, are disclosed in Japanese Patent Laid-Open Gazette 5-19891, Japanese Patent Laid-Open Gazette 5-303656, and Japanese Patent Laid-Open Gazette 4-329024.
In addition to the second demand, there is a third demand that an increase in cost is avoided by requiring no design changes of a sensor connected from outside the microcomputer 1. However, the second and third demands cannot be satisfied only by providing a difference in the voltage of power-supply between an I/O interface 6 and an internal logic circuit 14.